The invention relates to an apparatus for determining the fuel injection quantity, the angle of ignition dwell, the exhaust gas recycle rate or similar parameters, in mixture compressing internal combustion engines. The invention relates to engines in which the injected fuel is supplied to combustion chambers of cylinders by injection valves which operate in dependence on the throttle valve position and on the r.p.m. of the engine. A set of characteristic curves of the duration of fuel injection as a function of r.p.m. with the throttle valve position as a variable parameter is provided for any particular type of internal combustion engine in a digitally coded computer circuit. The instantaneous values of the engine r.p.m. and of the throttle valve position are also digitally coded and are supplied to the computer circuit. The computer circuit includes a memory with a predetermined number of storage cells from which the circuit is able to derive the appropriate fuel injection duration based on the instantaneous values of r.p.m. and throttle valve position.
Mixture compressing internal combustion engines must be supplied with the proper amount of fuel corresponding to the aspirated air quantity for each power stroke of the engine. The amount of fuel must be such that the combustion produces adequate power but operates without an excess of fuel since that would result in an intolerably high degree of toxic components.
For these reasons, it is desired to supply a combustion fuel-air mixture which is either at the stoichiometric ratio, where the air number .lambda. is equal to 1 or lies in a region in which there is excess air; the latter condition is particularly suitable to reduce toxic exhaust gas components up to a certain limit so as to permit compliance with the evermore rigorous requirements with respect to atmospheric purity. Only during full-load operation (fully open throttle valve) is it necessary to operate the engine with air numbers where .lambda. is less than 1. However, in order to correctly adjust the duration of fuel injection, for example when the fuel is supplied to the cylinders or to the induction tube via injection valves, it is necessary to know the aspirated air quantity exactly. This knowledge may be derived from measurement of the air flow rate in the induction tube of the engine, for example by means of a baffle plate which is displaced against a restoring force and serves to adjust appropriate metering means coupled thereto. Unfortunately, this is a relatively expensive process which, furthermore, suffers from the inherent disadvantage that when the throttle is opened, the increase of the engine torque is delayed due to the delay between the increased aspirated air quantity with regard to the newly set throttle valve position.
Instead of making an air flow rate measurement, it is also possible to fix the fuel injection duration on the basis of the engine r.p.m. and the induction tube pressure. By following the characteristic curve of an induction tube pressure sensor, the correct amount of fuel as a function of induction tube pressure for a particular r.p.m. may be determined.
Induction tube pressure measurements are however also quite complicated and, just as in the baffle plate measurement, additional sensors are required. Furthermore, as in the air flow rate measurement, there is a delay in the occurrence of the torque increase. A supplementary mechanism is required to achieve a temporary enrichment during a change of the throttle valve position so as to obtain a good transition from one state to the next.
It is also known to determine the fuel quantity supplied to the combustion chambers in the cylinder, i.e., to determine the injection time when using injection valves supplied with fuel at a certain pressure from the instantaneous values of the throttle valve position and the r.p.m. These two values alone are suitable to make an unambiguous determination of the fuel quantity to be injected.
However, this method requires to have available a so-called set of characteristic curves for each and every type of engine which is to be supplied with fuel. This set of characteristic curves shows the dependence of the fuel quantity to be injected or of the injection duration t.sub.i as a function of the r.p.m., with the common parameter being the throttle valve position. A known characteristic set of curves for a process of this type is shown schematically in FIG. 2 and will be discussed in more detail below. When mechanical injection systems are used, the solution involves a three-dimensional cam which determines the fuel quantity to be injected on the basis of the prevailing values of the r.p.m. and the throttle valve position. As may be seen from the curves in FIG. 2, the fuel injection quantity depends in a relatively complicated manner on the r.p.m. and the throttle valve position. For this reason, it has heretofore been regarded as impossible to simulate the function which defines the injected fuel quantity with any reasonable amount of effort and expenditure in an electrical or electronic injection system. In the function t.sub.i = f (.alpha. , n) shown in FIG. 2, t.sub.i is the time during which fuel is injected to a cylinder per power stroke and is therefore proportional to the fuel quantity Q. .alpha. is the instantaneous position of the throttle valve and n is the instantaneous r.p.m. Since the above-mentioned function f is difficult to follow in a direct manner, a known circuit uses a low pass filter and a pulse shaping circuit to transform this function into a somewhat simpler function which is easier to follow, and this simpler function is subsequently multiplied or modulated by another r.p.m.-dependent function. This known method also entails a substantial expense.